Experimental investigation of lean-dome high-airflow airblast pilot mixers' operability, emissions, and dynamics

Abstract

In order to further improve operability and low-power combustion efficiency of single-venturi twin concentric swirlers Lean Direct Injection (LDI)-based lean dome combustion systems, the authors' previous work investigated the effect of outer swirler vane angle on LDI reacting flow characteristics. A typical single-element LDI injector comprises of a fuel atomization device (pressure injector or airblast), surrounded by high airflow twin swirlers with converging passages feeding a single venturi followed by a single diverging passage called flare. LDI injectors without flare can be classified as high airflow airblast or air-assist pressure atomizers, in contrast with conventional air atomizing devices where air-to-fuel ratio rarely exceeds 3 or 4. In the present investigation, the impact of the flare section on the reacting flow field and flame structure is systematically explored. Specifically, the lean blowout limits, flame shapes, reacting flow dynamics, and NOx emissions from swirl-venturi LDI and airblast injectors are experimentally characterized using a suite of optical diagnostics, including time-resolved particle image velocimetry, OH⁎/CH⁎/NO2⁎ chemiluminescence, and broadband flame imaging. Results reveal that the inclusion of a flare significantly increases the overall swirl strength of the injector. Conversely, by removing the flare, swirl strength is reduced, leading to flow pattern changes (such as a transition from recirculating to swirling jet flow), higher lean blowout limits, and higher peak oscillation frequency of the flow field, while NOx emissions decrease due to reduced characteristic residence times. Such results demonstrate an inherent design tradeoff between lowering NOx emissions and maintaining/enhancing operability for a single swirl injector.